Because it's hard, not a lot of people will like or respect you for it, but it has to be done, and you have a chance of making a positive difference by protecting and serving your community.
At least that's my reason if I were one. Due to my asthma and other health issues, I will never be a cop, but I plan to work alongside officers, which is why I am pursing the legal sector. If health wasn't an issue, I'd be a cop.
I have great respect for my local police. They've helped me out so much. I don't associate bad apples with the whole force, I'm better than that. And yes, I've dealt with crooked officers myself, but my view of the police as a whole isn't soured by it.
Want the force cleaned up? Get more good people who want to serve their community in the force instead of demo isn't it as a whole. It's the only way things can get better, by getting more good people in it. Disrespecting the risk and sacrifice many of these officers put on the line every day is not helping any good people inserted in the force to actually join.
Why would anyone want to be a cop? Everyone has a different answer, but when someone's answer is to serve and help local communities and people in the neighborhood, we should be encouraging them, because we need more good people who don't crave power on the force!